It is generally known that anhydrous sodium metasilicate can be produced through fusion or sintering of soda with sand. The fusion or sintering product thus obtained has to subsequently be crushed, and so the final product unavoidably contains considerable amounts of dust. There are also substantial difficulties when one attempts to obtain a crystalline product through the evaporation of sodium metasilicate solutions. Here, the heat exchange surface will become coated, in a short period of time, with a hard layer of anhydrous sodium metasilicate, which coating then precludes any economical heat transfer. Because of the high temperatures that are required, there are also certain difficulties in obtaining anhydrous products by atomizing sodium metasilicate solutions. In addition, such spray-dried products contain substantial amounts of dust.
To avoid the above-mentioned disadvantages, it has been suggested in, for example, German published application No. 15 67 572, to produce anhydrous crystalline alkali metal silicate, preferably sodium metasilicate, by spraying an aqueous alkali metal silicate solution onto a finely distributed solid alkali metal silicate, which is heated to a temperature in excess of 130.degree. C. and kept in constant motion, thus forming a film of said aqueous alkali metal silicate solution on the surface of said solid alkali metal silicate, and evaporating water by means of an additional hot gas stream, whereby the spraying and drying steps are repeated until the crystalline anhydrous alkali metal silica particles have grown to the desired size. Generally, a portion of the resulting largely anhydrous alkali metal silicate is returned to the continuous reaction as starting component.
According to German published application No. 21 00 544 anhydrous sodium metasilicate is produced by injecting, through jet nozzles, a from 40 to 60 percent by weight sodium metasilicate solution into a highly turbulent zone, where the sodium metasilicate grains can be maintained at a temperature of between 125.degree. and 350.degree. C. The turbulent zone is created by a rising hot air stream which has a speed of between 22.9 and 61.0 m/minute.
The disadvantages of these known procedures are that in the sodium metasilicate being in constant motion in the turbulent zone a large number of sodium metasilicate grains is required for seeding and so a large portion of the already obtained spray granules have to be crushed again and returned to the turbulent zone, so that the overall yield of this procedure is being reduced.
From German Pat. No. 968,034, it is also known how to produce solid, water of crystallization-containing, sodium metasilicate by homogeneously mixing finely distributed silicic acid, such as quartz sand or quartz meal, with aqueous sodium hydroxide, at a ratio which corresponds approximately to the alkali metal oxide to SiO.sub.2 ratio of the product to be made, adding this mixture continuously into a tubular reactor, against the pressure inside the reactor, and passing it through the tubular reactor in such a way at temperatures maximally about 175.degree. C., so that an even distribution of the silicic acid within this mixture is being assured by regulation of the linear throughput speed. Subsequently, the hot reaction product is taken out of the tublar reactor through a reduction valve, whereby the originally higher water content of the reaction product formed is lowered through water evaporation to the desired water content of the end product. This way it is possible to produce sodium metasilicate hydrate with less than 9 mols of water of crystallization.
It is further known from European patent application No. 0 001 368 to produce a metasilicate containing from 2 to 6 percent by weight of water by injecting an aqueous metasilicate solution into a pulverized, stirred mass of an anhydrous metasilicate, whereby the mixing zone is heated with hot gases to the extent that the granulate thus formed is given sufficient consistency and it also retains from 1 to 8 percent by weight of water in the granulate. Subsequently the granulate obtained is exposed to a heat treatment in a stirred container to accomplish an at least partial conversion into metasilicate penta-hydrate.
Furthermore, Netherlands paatent application No. 78 02 697 discloses the production of sodium silicate solutions by passing sand together with sodium hydroxide, under elevated pressure and at a temperature of at least 200.degree. C., through a tubular reactor such as one used for the continuous decomposition of bauxite. Such a reactor is known, for example, from German published application (DE-OS) No. 21 06 198 as well as from German published application (DE-OS) No. 25 14 339. During the production of metasilicates one preferably works at a temperature of from 200.degree. to 240.degree. C.; for products with a higher ratio of SiO.sub.2 :Na.sub.2 O one preferably uses a temperature range of from 240.degree. to 280.degree. C. The pressure inside the reactor is preferably in the range of from 100 to 200 bar. However, according to the procedure described, only solutions and not solid products are obtained.